The present invention relates generally to producing a High Temperature Thin Film Layer On Glass (HTTFLOG) of silicon, which is a precursor component of thin film transistors (TFTs) or other electronic devices. The term thin film is used in this invention to describe layers of less the 50 microns thickness.
The invention described here is a superior method to fabricate HTTFLOG precursor structures or components for electronic devices, including liquid crystal displays (LCDs). These displays have quicker production time and lower cost of manufacture, while enabling a groundbreaking increase in small and large screen resolution. The invention described here is a new sub-assembly intended for original equipment manufacturer (OEM) consumption and inclusion in electronic products.
One of the inventors of the present invention has patents directed toward semiconductor poly-silicon solar cells (U.S. Pat. Nos. 6,180,871, 6,320,117, 6,509,204). These patents describe a structure that includes a p-n junction diode. The p+ and n− polycrystalline silicon structures making up the PIN junction are formed using an Excimer laser. An advantage of using the Excimer laser (XeCl) is that it may form the polycrystalline silicon without destroying a low melting point substrate upon which the solar cell is fabricated.
The present invention teaches an efficient method of component manufacture based on incorporating the HTTFLOG fabrication into the widely used, high temperature, Float Glass and other comparable glass manufacture processes, without the need for the Excimer or other comparable laser.
Background on the Float Glass Process
The flat glass industry and its primary products are classified under Standard Industrial Classification (SIC) 3211. Among the products included are flat building glass, cathedral glass, float glass, antique glass, sealed insulating glass units, laminated glass made from glass produced in the same establishments, picture glass, plate glass (rough or polished), skylight glass, flat structural glass, tempered glass, window glass, etc.
In the Float Glass process, the batch raw materials are melted in very large furnaces and the exiting molten glass is “poured” onto a pool of molten tin. A continuous ribbon of glass is then drawn from this spreading mass.
Prior Art
The low temperature thin film silicon layer forming process, commonly called the low temperature approach (LTPSTFT, low temperature poly-silicon TFT) in the industry is normally limited to 450° Celsius. This is a temperature limit that is commonly recognized within the industry. The temperature range limit is used in order to not melt or distort the substrate surfaces.
Both the prior art low temperature approach and the high temperature approach, HTTFLOG introduced here, can form precursors for transistors, solar cells, and other devices with polycrystalline silicon (poly-silicon). As is well known in the art, polycrystalline silicon can be formed by heating silicon to its melting point and allowing it to cool. Typically, silicon begins to crystallize after it melts at temperatures greater than about 1400° C. and begins to cool below that level. High temperature processes are not appropriate for substrates made of plastics or other materials that melt at low temperatures.
One important element in the cost/performance formula in the manufacture of thin film transistor (TFT) or other electronic devices is the use of silicon. Presently, cost/performance tradeoffs have led to two approaches in the industry for the makeup of the devices. One is amorphous silicon (a-Si) and the other is polycrystalline silicon (pc-Si).
Amorphous silicon can be deposited using low temperature processes. It can be deposited and formed on plastic and other flexible substrates and also be formed over large surfaces. This processing technique is less expensive than a poly-silicon process. Nevertheless, amorphous silicon transistors introduce other significant limitations not found in crystalline silicon or polycrystalline silicon transistors. For example, hydrogen is generally added during the manufacturing to increase the efficiency. However, amorphous silicon transistors tend to lose this hydrogen over time, causing reduced efficiency and reduced usable life.
Pc-Si is by far the preferred technical solution because of the speed and performance of the transistor. A-Si is still used (but diminishing) because it produces lower performance transistors, but they are historically less expensive to manufacture.
The low temperature approach for pc-Si normally uses laser annealed a-Si to get the best performance, but it is an expensive approach. There is a niche market in which it is necessary: LCD projection and small display devices for cameras, cell phones, iPads, and electronic tablets. These employ small high-resolution displays which are made by low-temperature methods today. Amorphous silicon displays today have not generally achieved the very high resolutions wanted for these devices by the marketplace.
In the manufacture of flat panel displays, it is common to use Excimer lasers to form polycrystalline silicon thin film transistors, so as not to distort the substrate. Laser annealing is an effective method of precisely melting the silicon layer without damaging the substrate, but it has other drawbacks.
Laser annealing, with Excimer laser type, equipment, is an expensive undertaking because it requires a large vacuum chamber, about 3 meters square, with interlocked entrance and exit apertures, and an internal moving stage. This is complex, sensitive, power hungry, and manpower intensive equipment. The exact positioning of the laser beam is unforgiving, and these lasers don't last long requiring maintenance, production downtime, and replacement.
Silicon has an energy absorption peak at about 308 nm wavelength. When using an Excimer laser it requires a very short pulse of about 45 nanoseconds to melt the thin film of silicon without heating the underlying glass substrate. This may be pulsed in the 30 Hz/sec. range.
Processing time is an important consideration in production cost. An Excimer laser is usually stepped over the surface, in small increments, generally at a rate of about 30 pulses per second. These steps are usually overlapped. They operate this way, and not more rapidly, to avoid overheating the substrate. They achieve accurate localized heating. The number of steps required when using a laser, the cycle time, and the material and tool handling considerations increase the expenses involved in the use of the low temperature pc-Si approach. In addition, the low temperature method of stepping the laser over the same area repeatedly causes fairly wide variations in the thin film poly-silicon surface. This results in wide variations in final transistor performance. Beta, the speed of the transistors, sometimes has a variation of 50%.
Current flat panel screens are of limited resolution. Most in the marketplace today are in the 200 pixels per inch (ppi) range. The very best in the marketplace today is 326 ppi, in the Apple Iphone 4. The screen in that device is small, measuring about four inches long. This screen resolution is an advertised technology leadership feature by Apple, as a significant market advantage.
An alternative to the low temperature pc-Si approach is the high temperature pc-Si approach, introduced in this invention.